Zusammenfassung

Preferential crystallization is an effective and comparatively cheap technology for enantioseparation at different scales. It is based on different crystallization rates of the two enantiomers in the presence of seed crystals of a single optically active isomer and thus allows the selective crystallization of just one enantiomer in a certain time period. The work to be presented here focuses on new concepts for performing preferential crystallization separations. The application of online polarimetry combined with refractometry and the microscopic investigation of the solid phase provides information on the crystallization kinetics [1]. Experimental results under different crystallization conditions (supersaturation, temperature and enantiomeric excess) in a batch crystallizer will be presented. Furthermore, we will show preliminary experiments in which the influence of the counter-enantiomer on the crystal growth of the desired enantiomer is investigated. Common goals for the control of crystallization processes are to obtain a product with prescribed distribution. In the case of preferential crystallization, an additional requirement is the purity of the final product. When preferential crystallization is performed in a cyclic operation mode, the initial state of each cycle depends on the final state of the previous cycle. The difference in the mass fractions of the two enantiomers will vary from cycle to cycle. The control problem then is to determine the optimal temperature cooling profile and the optimal mass of the added seeds and racemate. The concept of orbital flatness constitutes a highly promising approach to this problem [2].
[1] M.P. Elsner, Chirality 2005
[2] U. Vollmer, International Journal of Control 2003, 76